The structure of undecanethiol self-assembled monolayers (SAMs) on Au(111) with -COOH and -OH tail groups has been characterized by combining experimental and theoretical techniques. The pure and mixed 1:1 SAMs were investigated. It was found using polarization modulation infrared reflection-absorption spectroscopy, X-ray photoelectron spectroscopy (XPS), and ab initio calculation methods that the chains adsorb via their S atom on two different sites (one near atop and another near bridge site) of the Au(111) surface and that inter-tail-group H bonds are formed, leading to chain paring. Only some traces of aggregation between the different chain types were observed, confirming our former atomic force microscopy results. The vibrational frequencies and XPS results obtained via both experimental and theoretical approaches have enabled to describe and understand the adsorbed mixed and pure thiol SAMs.